I am going to brew a german pils, with decoction.
I wanted to use in the grain bill just 100% of german pils malt, but beerools recipe calculator says, that I must add some darker malts (munich, crystal?) What are your suggestions?

Beer tools is just that... a tool to help you develop a recipe within a style based on gravity, color, IBUs and alcohol level. End state you can build your recipe any way you would like. It may or may not turn out the way you planned but it's a hobby and we learn more from mistakes than successes. I'm not saying that your plan is destined for disaster it is just not within the parameters of the style.